In machine technology and especially in the construction of coordinate measuring apparatus, it is often necessary to protect longitudinally extending, movable components which project into the work or measuring area against unintended collisions. Such machine components can be, for example, the measuring arm of the coordinate measuring apparatus which carries the measuring tool (probe head). Injury to operating personnel by the movable machine components is also to be prevented.
For this purpose, various protective devices are already known. For example, U.S. Pat. No. 4,713,892 discloses a protective device wherein an ultrasonic envelope is placed about the machine component to be protected. On the other hand, European patent publication 0,116,807 as well as German published patent application 3,514,444 disclose protective devices in the form of several light barriers which extend along the length of the machine component. The protective envelope is provided by a plurality of light barriers arranged one next to the other. When this protective envelope is penetrated, the electronic circuit to which the light barriers are connected supplies a signal to the emergency-stop control of the particular machine.
These known protective arrangements do operate very reliably; however, a plurality of light barriers arranged close one next to the other is required to provide the reliability of operation which is adequate. This configuration is needed since no gaps can occur in the protective envelope defined by the light barriers through which, for example, projections of the object to be measured can come into contact with the measuring arm and damage the latter.
It is also known to provide a plurality of spiral springs around the measuring arm of coordinate measuring apparatus to provide protection for this vertical arm. The spiral springs are at an electrical potential other than the metal machine component and the protective switch of the machine responds when there is contact with the machine component, for example, as in the course of a collision. This solution is however less suited for horizontally disposed machine components since the springs droop there and tend to oscillate thereby triggering false switching operations. Moreover, this solution is not usable for machine components made of aluminum having anodized surfaces because the non-conductive anodized surface prevents the electric short circuit between spring and machine component.
A protective device for the measuring arm of a coordinate measuring apparatus is described in U.S. Pat. No. 5,038,488 wherein two light barriers are guided through cutouts in the partition legs of a bellows surrounding the machine component. A displacement of the partition legs in the case of a collision interrupts the light barrier and the change of the light intensity announced by the detectors is processed further as a signal for the emergency control of the machine.
However, existing machines cannot easily be equipped with this protective device as larger bellows are required because of the necessary minimum dimensions for the light sources, detectors, the beam cross section etcetera and the measuring arm of the machine would therefore take up more space and the machine modified in this way could no longer follow the movement sequence programmed on the basis of the thinner arm. Furthermore, it is not simple for light barriers to maintain the beam direction and the beam intensity adequately constant over a long time duration.